1. Field of the Invention
This invention relates to printed circuit boards (PCB), and more particularly to a PCB having permanent solder mask.
2. Description of the Prior Art
The U.S. Pat. No. 5,626,774 discloses a method for forming a solder mask on the surface of a PCB, which introduces a new idea that the material of the solder mask can be an epoxy resin selected from those used in the PCB field since it will be generally compatible with the glass fiber reinforced epoxy substrates. However, the method does not teach what a proper solder mask an outstanding PCB must have. For example, the outer surface of the solder mask must smooth enough, if not, there will need a lot of underfills between the outer surface and the chip attached thereon. In addition, if the roughness of the outer surface of the solder mask is unappropriate, the solder mask and the encapsulation of the IC chip package cannot be firmly combined each other.
Moreover, the conductive pattern disposed on the surface of the prior art PCB often includes a plurality of exposed portions uncovered by the solder mask for electrically connecting with other electrical devices. The adjacent wiring of the exposed portions will be shorted frequently due to the ion migration therebetween.
Furthermore, the vias of the prior art PCB, which run through the PCB and are used to electrically connect the conductive patterns disposed respectively on the top and bottom surfaces of the PCB, are not filled with the solder mask, so during the baking process of the PCB, the air contained in the vias will induce internal stress between the solder mask and the PCB. Such internal stress often results in the bend of the PCB.
It is an object of the present invention to provide an improved PCB having a permanent solder mask that has an even and smooth outer surface.
Another object of the present invention is to provide an improved PCB including a permanent solder mask on which an outer surface with an optimum micro-roughness is formed.
A further object of the present invention is to provide an improved PCB on which a conductive pattern having a high-density wiring can be disposed.
These objects are achieved with a printed circuit board (PCB) having a permanent solder mask. According to one aspect of the present invention, the PCB includes a substrate made of a material including a first resin. At least a conductive pattern is disposed on one surface of the substrate. A second resin having a thermal expansion coefficient substantially identical to that of the first resin is coated on the surface having the conductive pattern such that a solder mask having a predetermined thickness and an even and smooth outer surface is formed. In the meantime, the conductive pattern is divided into a sheltered portion covered by the solder mask and an unsheltered portion exposed outside. The even and smooth outer surface of the substrate can be defined in such a way that the difference between the thickness of the highest position of the solder mask and that of the lowest position of the solder mask is equal to or less than 10 xcexcm measuring with a length unit of 500 mm.
According to a second aspect of the present invention, a PCB having a permanent solder mask is provided. The PCB comprises a substrate made of a material including a first resin. At least a conductive pattern is disposed on one surface of the substrate. A second resin having a thermal expansion coefficient substantially identical to that of the first resin is coated on the surface having the conductive pattern in such a way that a solder mask having an outer surface with a micro-roughness ranging between 0.5 xcexcmxcx9c10 xcexcm is formed.
According to a third aspect of the present invention, a PCB having a permanent solder mask is provided. The PCB comprises a substrate made of a material including a first resin. At least a conductive pattern having a sheltered portion and an unsheltered portion disposed on one surface of the substrate. The unshelteed portion of the conductive pattern has a base and a tip. A solder mask made of a second resin having a thermal expansion coefficient substantially identical to that of the first resin is coated on the surface of the substrate in such a way that a first area having a predetermined thickness and an even and smooth outer surface and a second area having a thickness less than that of the first area are formed. The first area of the solder mask covers the sheltered portion of the conductive pattern. The second area surrounds the base of the unsheltered portion such that the tip of said unsheltered portion is exposed outside.